Tooling

Tools/Terrain/internal/dt/dt_test.go

@@ -0,0 +1,116 @@
+package dt
+
+import (
+	"math"
+	"testing"
+)
+
+func scatter(w, h int, seed uint32) []bool {
+	seeds := make([]bool, w*h)
+	for i := range seeds {
+		seed = seed*1664525 + 1013904223
+		seeds[i] = seed>>20&7 == 0
+	}
+	seeds[0] = true // guarantee at least one
+	return seeds
+}
+
+// brute is the definition: the smallest squared distance to any seed, with dx measured the short way round
+// when the grid is a cylinder.
+func brute(seeds []bool, w, h, x, y int, wrapX bool) float64 {
+	best := math.Inf(1)
+	for sy := 0; sy < h; sy++ {
+		for sx := 0; sx < w; sx++ {
+			if !seeds[sy*w+sx] {
+				continue
+			}
+			dx := float64(x - sx)
+			if wrapX {
+				if d := math.Abs(dx); d > float64(w)/2 {
+					dx = float64(w) - d
+				}
+			}
+			dy := float64(y - sy)
+			if d := dx*dx + dy*dy; d < best {
+				best = d
+			}
+		}
+	}
+	return best
+}
+
+func check(t *testing.T, w, h int, wrapX bool) {
+	t.Helper()
+	seeds := scatter(w, h, 99)
+	d2, near := Transform(seeds, w, h, wrapX)
+	for y := 0; y < h; y++ {
+		for x := 0; x < w; x++ {
+			want := brute(seeds, w, h, x, y, wrapX)
+			i := y*w + x
+			if math.Abs(float64(d2[i])-want) > 1e-3 {
+				t.Fatalf("wrap=%v cell (%d,%d): d2 %g, brute force %g", wrapX, x, y, d2[i], want)
+			}
+			// The feature index must be a seed, and it must be one at exactly that distance.
+			n := int(near[i])
+			if n < 0 || !seeds[n] {
+				t.Fatalf("wrap=%v cell (%d,%d): nearest %d is not a seed", wrapX, x, y, n)
+			}
+			got := brute(onlyAt(w, h, n), w, h, x, y, wrapX)
+			if math.Abs(got-want) > 1e-3 {
+				t.Fatalf("wrap=%v cell (%d,%d): nearest seed %d is at %g, not %g", wrapX, x, y, n, got, want)
+			}
+		}
+	}
+}
+
+func onlyAt(w, h, i int) []bool {
+	s := make([]bool, w*h)
+	s[i] = true
+	return s
+}
+
+// The one test the coastal pass rests on. Everything there is written in terms of "how far is this cell from
+// the waterline and which stretch does it belong to", so a distance transform that is subtly wrong would not
+// fail loudly - it would put the shelf break in slightly the wrong place everywhere. The transform is exact,
+// so the comparison is against an exhaustive search and the tolerance is float32 rounding.
+func TestMatchesBruteForce(t *testing.T) { check(t, 41, 37, false) }
+
+// And the same on a cylinder, which is what a planet is. The failure this catches is a shelf that stops dead
+// at the seam.
+func TestMatchesBruteForceOnACylinder(t *testing.T) { check(t, 41, 37, true) }
+
+// A seed on one edge must be found from the other edge, and by the short way round.
+func TestWrapFindsTheSeedAcrossTheSeam(t *testing.T) {
+	const w, h = 9, 3
+	seeds := make([]bool, w*h)
+	seeds[h/2*w+0] = true // one seed, at column 0 of the middle row
+
+	d2, near := Transform(seeds, w, h, true)
+	// Column 8 is one step from column 0 the short way round, eight steps the long way.
+	if got := d2[h/2*w+8]; math.Abs(float64(got)-1) > 1e-6 {
+		t.Errorf("d2 at column 8 = %g, want 1", got)
+	}
+	if got := near[h/2*w+8]; got != int32(h/2*w) {
+		t.Errorf("near at column 8 = %d, want %d", got, h/2*w)
+	}
+	// The far side of the cylinder is four steps away either way.
+	if got := d2[h/2*w+4]; math.Abs(float64(got)-16) > 1e-6 {
+		t.Errorf("d2 at column 4 = %g, want 16", got)
+	}
+	// Without the wrap the same grid gives eight.
+	d2f, _ := Transform(seeds, w, h, false)
+	if got := d2f[h/2*w+8]; math.Abs(float64(got)-64) > 1e-6 {
+		t.Errorf("unwrapped d2 at column 8 = %g, want 64", got)
+	}
+}
+
+func TestNoSeedsAtAll(t *testing.T) {
+	const w, h = 5, 4
+	seeds := make([]bool, w*h)
+	_, near := Transform(seeds, w, h, true)
+	for i, n := range near {
+		if n != -1 {
+			t.Fatalf("cell %d reports a nearest seed %d on an empty grid", i, n)
+		}
+	}
+}